Optical structure and method for its production

ABSTRACT

An optical structure which is suitable as an optical waveguide or cavity comprises a carrier having a lattice structure with a photonic band gap and a defect region. The lattice structure comprises pores which have constriction and are arranged in a periodic grid pattern which is disturbed in the defect region. The optical structure can be produced by the electrochemical etching of silicon.

BACKGROUND OF THE INVENTION

In optical waveguides, which for example, are used for optical datatransmission, and cavities, which, for example, are used on laserresonators, the propagation of light is limited in at least two spatialdirections. In this case, the wave guidance usually takes place by totalreflection at the interface between an optically denser and an opticallyless dense medium. The light propagates in the optically denser mediumin this case.

Recent scientific papers are concerned with the propagation of light inperiodic dielectric lattice structures. The propagation of light instructures of this type can be described analogously to the propagationof electrons in a crystal. If the wavelength of the light is of theorder of magnitude of the dimensions of the lattice, then a photonicband gap can form. The photonic band gap is a frequency range in whichphotons cannot propagate. This means that if light at a frequency whichlies in the frequency range of the photonic band gap is radiated onto astructure of this type, then this light cannot propagate in thestructure. Instead, it is reflected at the surface. This effect has beenconfirmed by experiments (see, for example, an article by E.Yablonovich, "Photonic Band Gaps and Localization", ed. C. M. Soukoulis,Plenum, New York, 1993, pages 207 to 234, or an article by U. Gruning etal., Appl. Phys. Lett., Vol. 66, No. 24, 1995, pages 3254 to 3256). Thisreflection is also referred to as a Bragg reflection at the dielectriclattice.

The experimental investigations were carried out on structures in whichthe lattice structure is realized as a layer structure havingalternating layers with a different refractive index or from anonmetallic material such as, for example, AlGaAs or GaAs or Si havingpores arranged in a periodic grid pattern. In AlGaAs and GaAs, thesepores are produced by reactive ion etching. In silicon, these pores havebeen produced by electrochemical etching.

On the basis of theoretical considerations and calculations, it has beenproposed to utilize the effect of the Bragg reflection at the dielectriclattice for the purpose of realizing cavities and optical waveguides(see, for example, an article by R. Maede et al., J. Appl. Phys., Vol.75, No. 9, 1994, page 4753). In this case, two regions made of amaterial having a photonic band gap are used for an optical waveguide.GaAs with a periodic hole structure has been proposed as a material forthe photonic band gap. As the optical waveguide, the starting material,GaAs, without hole structures is arranged between the two regions. Inthis optical waveguide, light having a wavelength which corresponds to afrequency in the photonic band gap is guided in a plane by virtue of thefact that it cannot propagate into the material having the photonic bandgap. In the plane perpendicular to this, the light is guided by totalreflection at the interface between the optically denser GaAs and thesurrounding, optically less dense atmosphere. In order to realize acavity, material having a photonic band gap is provided for the purposeof limiting the propagation of the light in the third spatial direction.

SUMMARY OF THE INVENTION

The invention is based on the problem of specifying a further opticalstructure which is suitable as an optical waveguide or cavity and inwhich the light propagation is prevented in at least one spatialdirection by means of the Bragg reflection at the dielectric lattice.Furthermore, it is intended to specify a method for producing such anoptical structure.

This problem is solved according to the invention by means of an opticalstructure having a carrier with a main surface and a lattice structurewith a defect region, said lattice structure being formed by poresextending perpendicular to said main surface and being arranged in aperiodic grid pattern outside of said defect region and said gridpattern being disturbed in said defect region, said lattice structurehaving at least one frequency band for light in which light of afrequency of said band will not propagate in the lattice structure, saidlattice structure having at least three regions arranged one above theother and extending parallel to said main surface, with the diameter ofthe pores in the second or middle region being smaller than the diameterof the pores in the outer or first and third regions.

To form the optical structure, the invention includes a method whichcomprises the steps of providing an n-doped silicon carrier having amain surface, forming a periodic grid pattern with a defect region ofdepressions in the main surface of the silicon carrier, and thenelectrochemically etching with at least three steps at the depressionsof the main surface with the main surface in contact with an electrolyteand the silicon carrier being connected as an anode and an etching ratebeing influenced by a setting of a current density with an etching of afirst value of current density for a first etching step to start etchinga pore for each depression with a first region, etching at a secondvalue of current density to form a second region of each pore and thenetching at a third value of current density to form a third region foreach pore, with the second value of the current density being less thanthe first and third values.

Further refinements of the invention emerge from the remaining claims.

In the optical structure according to the invention, a lattice structureis provided in a carrier. Any material which is not metallic and doesnot absorb the light with which the optical structure is to be operatedis suitable for the carrier. In particular, the carrier is realized froma III-V semiconductor or from silicon.

The lattice structure has a photonic band gap, that is to say it has theproperty that there is at least one frequency band such that light at afrequency from this frequency band cannot propagate in the latticestructure. The lattice structure constitutes a dielectric lattice atwhich this light undergoes the Bragg reflection.

The lattice structure is formed by an arrangement of pores which runessentially perpendicular to a main surface of the carrier and have anessentially identical cross section. The cross section of the pores ispreferably round, but may also be angular, for example square. Outside adefect region in the lattice structure, the pores are arranged in aperiodic grid pattern. In the defect region, on the other hand, theperiodic grid pattern is disturbed. The disturbance may consist in analtered lattice spacing between at least two pores, in the absence of atleast one pore or in at least one pore which either is filled withanother material or has a different diameter.

The pores have a constriction in the direction extending perpendicularto the main surface. This means that there are three regions arrangedone above the other and aligned essentially parallel to the mainsurface, and the diameter of the pores is smaller in the middle regionthan in the two outer regions. As a result of this variation of thediameter of the pores, the dielectric constant and hence the refractiveindex the middle region differ from the dielectric constant and therefractive index in the outer regions. The middle region is thusoptically denser than the outer regions. In the direction perpendicularto the main surface, light is therefore guided by total reflection atthe interface between the middle region and the outer regions.Perpendicularly to this, the light is guided by virtue of the fact thatit cannot propagate into the lattice structure on account of itswavelength, since the lattice structure has a photonic band gap for thiswavelength. In the optical structure according to the invention, thelight is guided in the carrier below the main surface.

The form of the optical waveguide or the cavity is determined by thegeometrical form of the defect region. A cavity is formed by the defectregion being bounded by the lattice structure in two directions of themain surface. For an optical waveguide, the defect region is extendedfurther, with the result that the lattice structure is subdivided intotwo parts. In this case, the optical waveguide may run straight or beangular.

The light guided in the defect region can be finely tuned with regard toits wavelength and/or mode by way of the dimensioning of the defectregion perpendicularly to the propagation of the light. The narrower thedefect region is, the more distinct the selection of the guided light iswith regard to frequency and mode.

It lies within the scope of the invention for the pores in the carrierto have more than one constriction. Two or more optical waveguides orcavities, which run one above the other, are realized as a result.

Further properties of the optical structure can be established by way ofthe form of the periodic grid pattern. If the periodic grid pattern issquare, then the optical structure is suitable for guiding polarizedlight. This case results in the formation of photonic band gaps for thetwo polarization directions of the light which do not overlap. If, onthe other hand, the grid pattern is trigonal, then the photonic bandgaps for the two polarization directions of the light overlap and theoptical structure is suitable for wave guidance of unpolarized light.

The optical structure can be produced by stacking differently structuredlayers one on the other. It can, furthermore, be formed by anisotropicetching in a substrate, the etching being carried out from two oppositesurfaces. The constricted region is realized using a spacer technology,for example.

The optical structure is preferably produced on the basis of n-dopedsilicon by means of electrochemical etching. In this case, depressionsarranged in a periodic grid pattern are first of all produced in a mainsurface of an n-doped silicon substrate. The grid pattern has a defectregion in which at least one depression is absent. The electrochemicaletching is carried out in an electrolyte which preferably containsfluoride and is acidic and with which the main surface is in contact. Avoltage is applied between the electrolyte and the silicon substrate,with the result that the silicon substrate is connected up as an anode.As a result, minority charge carriers in the n-doped silicon move to themain surface which is in contact with the electrolyte. A space chargezone is formed at this main surface. Since the field strength is greaterin the region of depressions in the main surface than outside saidregion, the minority charge carriers preferably move to these points.This results in self-aligned structuring of the surface. The deeper aninitially small depression becomes as a result of the etching, the moreminority charge carriers move there owing to the increased fieldstrength, and the greater the etching attack is at this point. The poresgrow as the etching time lengthens.

The etching attack depends on the current density in the siliconsubstrate. By increasing the current density in the electrolyte, theetching attack is increased and the cross section of the pore is thusenlarged. The etching is carried out in at least three etching steps inthe method according to the invention. Etching is carried out with afirst value for the current density in the first etching step, etchingis carried out with a second value for the current density in a secondetching step and etching is carried out with a third value for thecurrent density in a third etching step. In this case, the second valuefor the current density in the second etching step is smaller than thefirst value for the current density in the first etching step and thethird value for the current density in the third etching step. As aresult, the pores are formed with a constriction. The constriction iseffected by the smaller, second value for the current density.

In order to produce the optical structure with pores which have two ormore constrictions, the electrochemical etching is correspondinglycarried out in five or more etching steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an optical structure having an opticalwaveguide.

FIG. 2 is a cross-sectional view through the optical structure havingthe optical waveguide which view is taken along line II--II of FIG. 1.

FIG. 3 is a plan view of an optical structure having at least onecavity.

FIG. 4 is a cross-sectional view through the optical structure whichview is taken along line IV--IV in FIG. 3, and shows the pores havingtwo constrictions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A carrier 1 made of n-doped, monocrystalline silicon comprises a latticestructure 2 (see FIG. 1 and FIG. 2). The lattice structure 2 is formedby a periodic arrangement of pores 4 which run essentially perpendicularto a main surface 3 of the carrier 1 and have an essentially round crosssection. In the lattice structure 2, there is a defect region 5 in whichthe periodic grid pattern is disturbed by virtue of the fact that nopores 4 are arranged in this region. The width of the defect region 5 isequal to the extent of one pore 4, for example.

The pores 4 are arranged in a trigonal grid pattern, for example. Thewavelength range of the light for which wave guidance occurs in thedefect region 5 is in this case set by way of the distance betweenneighboring pores 4. The relationship a/λ≈0.2 to 0.5 is generally truefor the distance a between the center points of neighboring pores andthe wavelength λ. By a corresponding arrangement of the pores in thegrid pattern, the wavelength of the guided light can be shifted throughthe entire wavelength range in which the material of the carrier 1 doesnot absorb. In the example where the carrier 1 is composed of silicon,this means that wave guidance can be set reliably in the wavelengthrange between 1.1 μm and 100 μm. For wave guidance in the wavelengthrange from 5 to 6 μm, the distance between neighboring pores 4 is a=1.5μm to 2.5 μm.

In a first region 6, which extends from the main surface 3 down into thecarrier 1, the pores 4 have a diameter of, for example, 2.2 μm with adistance between neighboring pores 4 of a=2.3 μm. In a second region 7,which is arranged underneath the first region 6, the pores 4 have adiameter of, for example, 2.0 μm, which is smaller than the diameter ofthe pores in the first region 6. In a third region 8, which is arrangedunderneath the second region 7, the pores 4 have a diameter of, forexample, 2.2 μm, which is a larger diameter than the diameter of thepores in the second region 7. The pores 4 do not have a constantdiameter throughout their depth, with the result that a constriction ofthe pores 4 occurs in the second region 7.

The effect of this constriction of the pores 4 is that more silicon ispresent in the second region 7 than either in the first region 6 or inthe third region 8. The second region 7 therefore constitutes a densermedium in comparison with the first region 6 and with the third region8.

Light having a wavelength λ which lies in the photonic band gap of thelattice structure 2 is guided in the plane perpendicular to the mainsurface 3 by virtue of the fact that it cannot propagate in the latticestructure 2 on account of the photonic band gap. In the directionperpendicular to this plane and parallel to the surface 3, this light isguided by total reflection at the interfaces of the second region 7 withthe first region 6 and with the third region 8. The sectional regionformed by the defect region 5 and the second region 7 acts as an opticalwaveguide.

In order to produce the optical structure, depressions are produced inthe main surface 3 of the carrier 1, which has a resistivity of 1 Ω cm,for example, which depressions are arranged in a periodic grid patterncorresponding to the pores 4. No depressions are produced in the regionof the defect region 5.

The depressions are produced, for example, after the production of aphotoresist mask with the aid of conventional photolithography andsubsequent alkaline etching.

After the photoresist mask has been removed, the main surface 3 of thecarrier 1 is brought into contact with a fluoride-containing, acidicelectrolyte. The electrolyte has a hydrofluoric acid concentration of 1to 50 percent by weight, preferably 3 percent by weight. An oxidizingagent, for example hydrogen peroxide, can be added to the electrolyte inorder to suppress the evolution of hydrogen bubbles on the main surface3 of the carrier 1.

The carrier 1 is connected up as an anode. A voltage of 0 to 20 volts,preferably 3 volts, is applied between the carrier 1 and theelectrolyte. The carrier 1 is illuminated with light from a rear sideopposite to the main surface 3, with the result that a current densityof, for example, 18 mA/cm² is set. Starting from the depressions, thepores 4 which run or extend perpendicular to the main surface 3 areproduced during the electrochemical etching.

After an etching time of, for example, 10 minutes, during which thecurrent density was set to be constant at the first value of 18 mA/cm²,the pores reach a depth of 10 μm, for example. The current density isthen reduced to a second value of, for example, 14 mA/cm² and theelectrochemical etching is continued at this value. In the process, thepores 4 grow further with a reduced diameter. The second region 7 of thepores 4 is formed. After an etching time of, for example, 5 minutes, thesecond region 7 of the pores 4 has a dimension perpendicular to the mainsurface 3 of 5 μm, for example. The current density is then increased toa third value of, for example, 18 mA/cm² and the electrochemical etchingis continued. In the process, the third region 8 of the pores 4 isproduced, in which region the diameter of the pores 4 is greater thanthe diameter of the pores 4 in the second region 7. After an etchingtime of, for example, 10 minutes, the third region 8 has an extentperpendicular to the main surface 3 of 10 μm, for example. The opticalstructure is thus completed.

A periodic lattice structure 2' is provided in a carrier 1', which, likethe carrier 1, is composed of n-doped, monocrystalline silicon. Theperiodic lattice structure 2' has a photonic band gap for light havingthe wavelength λ. The lattice structure 2' is produced by pores 4'formed in a main surface 3' of the carrier 1' (see FIG. 3 and FIG. 4).

The center points of the pores 4' are arranged in a periodic, trigonalgrid pattern. In this case, the distance a between neighboring centerpoints of the pores 4' satisfies the condition a/λ≈0.2 to 0.5. Owing tothis dimension, the lattice structure 2' has a photonic band gap forlight having the wavelength λ. In this case, for example, a=2.3 μm andλ=5 μm.

A defect region 5', in which the periodic grid pattern is disturbed byvirtue of the fact that a pore 4' is absent, is provided in the latticestructure 2'. Light having the wavelength λ cannot propagate in thedefect region 5' parallel to the main surface 3', since the propagationof the light having the wavelength λ is impossible in the latticestructure 2' owing to the photonic band gap of the surrounding latticestructure 2'.

The pores 4' do not have a constant diameter throughout their depth. Ina first region 6', which adjoins the main surface 3', the pores have adiameter of 2.2 μm, for example. In a second region 7', which isarranged underneath the first region 6', the pores have a smallerdiameter of 2.0 μm, for example. In a third region 8', which is arrangedunderneath the second region 7', the pores have a diameter of 2.2 μm,for example. In a fourth region 9', which is arranged underneath thethird region 8', the pores have a smaller diameter of 2.0 μm, forexample. In a fifth region 10', which is arranged underneath the fourthregion 9', the pores have a diameter of 2.2 μm, for example. The depthof the first region 6' is, for example, 10 μm, the depth of the secondregion 7' is 5 μm, the depth of the third region 8' is 20 μm, the depthof the fourth region 9' is 5 μm and the depth of the fifth region 10' is10 μm.

The cross section of the pores 4' has a constriction both in the secondregion 7' and in the fourth region 9'. As a result, there is moresilicon present both in the second region 7' and in the fourth region 9'than in the adjoining first region 6', third region 8' and fifth region10'. The second region 7' and the fourth region 9' therefore constitutesan optically denser medium than the respectively adjoining regions 6',8', 10'.

Light having the wavelength λ, which is prevented from propagating inthe defect region 5' in the extent parallel to the main surface 3' bymeans of the surrounding lattice structure 2', is held in the directionperpendicular to the main surface 3' by total reflection at theinterface with the optically less dense medium in the second region 7'and in the fourth region 9'. The sectional region between the defectregion 5' and the second region 7' as well as between the defect region5' and the fourth region 9' in each case constitutes a cavity.

The production of the optical structure which has been explained withreference to FIG. 3 and FIG. 4 is carried out in a manner analogous tothe production of the optical structure which was explained withreference to FIG. 1 and FIG. 2, by electrochemical etching. In order toform the fourth region 9' and the fifth region 10' of the pores 4', inthis case the electrochemical etching is continued further, the currentdensity for etching the fourth region 9' being set to the second value,which was used for etching the second region 7', and the current densityfor etching the fifth region 10' being set to the third value, which wasused for etching the third region 8.

The invention can be applied analogously to optical structures in whichmore than two optical waveguides or cavities are arranged one above theother. For this purpose, the number of constrictions of the pores iscorrespondingly increased.

We claim:
 1. An optical structure comprising a carrier with a mainsurface and a lattice structure with a defect region, said latticestructure being formed by pores extending perpendicular to said mainstructure and being arranged in a periodic grid pattern outside of saiddefect region and siad grid pattern being disturbed in said defectregion, said lattice structure having at least one frequency band forlight in which light of a frequency of said band will not propagate inthe lattice structure, said lattice structure having at least a first,second and third region being arranged one above the other and extendingparallel to said main surface with the diameter of the pores in thesecond region being smaller than the diameter of the pores in the firstand third regions.
 2. An optical structure as claimed in claim 1, inwhich the defect region subdivides the lattice structure into at leasttwo parts in the region of the main surface.
 3. An optical structure asclaimed in claim 1, in which at least one further region is providedunderneath the third region, the further region having a pore diametersmaller than the pore diameter in the region above and below the furtherregion.
 4. An optical structure as claimed in claim 1, wherein thecarrier is composed of silicon.
 5. An optical structure as claimed inclaim 4,wherein the distance between the center points of neighboringpores lies in the range between 0.5 μm and 25 μm, wherein the diameterof the pores in the second region lies in the range between 0.4 μm and23 μm, wherein the diameter of the pores in the first region and in thethird region lies in the range between 0.45 μm and 24.5 μm, and whereinthe extent of the second region perpendicular to the main surface isbetween 1 μm and 50 μm.
 6. An optical structure as claimed in claim 1,wherein the grid pattern is trigonal.
 7. A method for forming an opticalstructure, said method comprises the steps of providing an n-dopedsilicon carrier having a main surface, forming a periodic grid patternof depressions in the main surface of the silicon carrier with a defectregion in the pattern, electrochemically etching with at least threesteps the main surface of the carrier with the main surface being incontact with an electrolyte and the silicon carrier being connected asan anode and an etching rate being influenced by setting of a currentdensity, etching at a first value of current density for a first etchingstep to start etching a pore at each depression to form a first regionfor each pore, then etching at a second value of a current density toform a second region of each pore and then etching at a third value of acurrent density to form a third region for each pore, with the secondvalue of the current density being less than the first and third valuesso that the second region of each pore has a smaller diameter than thepore in the first and third regions.
 8. A method as claimed in claim 7,wherein the current density is set by illuminating a rear side, oppositeto the main surface of the silicon carrier.
 9. A method as claimed inclaim 8, wherein the silicon carrier is a <100> wafer, and theelectrochemical etching is carried out in a fluoride-containing, acidicelectrolyte.
 10. A method as claimed in claim 7, wherein the depressionsin the main surface are produced by providing a photoresist mask on themain surface and subsequently carrying out an alkaline etching of themain surface.
 11. A method according to claim 7, wherein the siliconcarrier is a <100> wafer, and the electrolyte is a fluoride-containing,acidic electrolyte.